1) Field of the Invention
In typical optical sights for firearms (Burris, U.S. Pat. No. 3,161,716) a principal optical sighting datum, or reticle, comprising a post, dot or crosshair is used to align the bore of the firearm along the longitudinal axis of pitch and the longitudinal axis of yaw with a target for purposes of enabling accurate aimed fire. Such optical sights typically incorporate no means for indicating a longitudinal axial rotation of the firearm.
Optical sights are typically mounted substantially above the plane of the bore of a firearm. The ballistic relationship between this configuration of bore and sight and the gravitational forces acting upon the projectile require that the line bisecting the longitudinal axis of bore and scope be exactly vertical relative to the gravitational forces acting upon the firearm for accurate aimed fire. During sighting, any axial rotation, or cant, of the firearm from this ideal position will induce significant roll axis error (Precision Shooting, June, 1991 pp 46-49).
Elimination of sighting errors due to roll axis rotational error in current and conventional practice is a matter of guesswork for the shooter.
2) Description of Prior Art
Benchrest shooters typically build massive, level benches and rest flat-bottomed firearms on plumbed mechanical rests as a means of eliminating roll axis error. Such means are not portable, rendering them ineffective for field or military use. Patent search has revealed proposed electrical and mechanical means for the elimination of cant (Beisner, U.S. Pat. No. 5,315,781; Parks, U.S. Pat. No. 5,005,308; Lichtenstern, U.S. Pat. No. 3,556,666 et. al.). Such means as are disclosed in prior art differ significantly from the present invention, as the following description will make perfectly clear.